Characterization of visible hydrophobic particles, more particularly visible hydrophobic particles in aqueous media is an important general problem, with a specific application in the pulp and paper industry, especially when dealing with secondary fiber. Deposition of stickies and tackies and formation of large agglomerates of hydrophobic materials are major obstacles in the manufacture of paper and tissue using recycled fiber. For paper grades, these non-polar, tacky contaminants, particularly when liberated during repulping, can become both undesirable components of papermaking furnishes and troublesome deposits on the mill equipment; for example, wires of the paper or tissue machine.
Stickies and tackies are organic materials that do not have a precise definition. Stickies and tackies are tacky substances contained in the pulp and process water system that deposit on paper/tissue machine clothing, cylinders, or rolls. Synthetic materials constituting stickies or tackies (a.k.a. white pitch) include adhesives and coating binders such as styrene butadiene rubber, ethylene vinyl acetate, polyvinyl acetate, polyvinyl acrylate, polyvinyl butyral, polybutadiene, etc., and components of printing ink such as wax, alkyd resins, polyol acrylates, etc. Natural wood pitch normally present in virgin pulp consists of fatty acids, fatty esters and rosin acids. Natural wood pitch is more polar than stickies, but, generally, also belongs to the class of hydrophobic admixtures and is similar in its properties relevant to contaminant monitoring.
The most common classification system of stickies used by the pulp and paper industry is based on size to categorize stickies into three classes: macro, micro and colloidal. Macrostickies are considered to be particles that result from primary disintegration of the recycled material during repulping. For classification purposes, macrostickies typically have a particle size greater than 0.1 mm. Macrostickies can be removed largely by coarse and fine screening.
Macrostickies are also those stickies that remain as screening residue after laboratory screening having a slot width of 0.10-0.15 mm. Major sources of these materials are hot melts and pressure-sensitive adhesives. Macrostickies can include adhesives and coating binders such as styrene butadiene rubber, ethylene vinyl acetate, polyvinyl acetate, polyvinyl acrylate, polyvinyl butyral, polybutadiene, and/or components of printing ink such as wax, alkyd resins, polyol acrylates, and other like substances. The methods of quantification of macrostickies that are currently practiced are laborious, and no continuous monitoring technique exists.
Micro (0.1-0.001 mm) and colloidal (<0.001 min) stickies are those that can pass the screening slots. The microstickies that pass the screens can later agglomerate and lead to deposits on the paper/tissue machine or pass into the product as newly formed secondary macrostickies.
This differentiation, based on size, is arbitrary and cannot be strictly applied to different monitoring methods. Thus, the size limitation of the proposed method is not the same as size limitations defining macrostickies, while those having skill in the art will recognize the method as one for macrostickies monitoring. In fact, in pulp and paper applications, the size of measured stickies can be smaller than 0.1 mm provided that the particles are detectable through an imaging system that may include a digital microscopic camera and optically differentiated from the surrounding medium. Colloidal stickies do not fit this definition, while a 0.05-0.10 mm fraction of macrostickies in fact does fit this definition. The size limits are generally defined by the capabilities of the imaging system used and the signal to noise ratio of the captured image, where the noise can be electronic as well as background from the surrounding medium.
Nalco has developed proprietary technology for monitoring microstickies based on quartz crystal microbalance (QCM) (see, e.g., Duggirala & Shevchenko, U.S. Patent Application Publication No. 2006/0281191; Shevchenko et al., U.S. Pat. No. 7,842,165). The QCM-based technology is suitable for microstickies but not macrostickies monitoring because the latter cannot be expected to stick to the QCM surface in the flow of the pulp slurry.
It is known that fluorescent dye molecules that emit from an intramolecular charge transfer state are sensitive to medium polarity. The dye molecules' fluorescence (both wavelength and intensity) is affected by the polarity as well in the case of non-homogeneous systems, such as suspensions of non-soluble organic liquids in water. These dyes can bind directly to hydrophobic particles or droplets that have a similar effect on the dyes' optical properties. Based on these properties, methods were developed to visualize protein stains based on dyes that bind non-covalently to protein complexes. These techniques have also been coupled with electrophoresis procedures.
Perfect et al. (WIPO Patent Application Publication No. WO2010007390) describes a method to assess multiphase (water/organic) fluid compositions that specifically monitor oil in water in the area of wastewater treatment. Perfect et al. identified Nile Red as a preferable molecule used to perform the method. Nile Red is ideal for assessment of the organic phase of a multiphase sample because it emits a more intense signal on contact with the organic phase than on contact with the aqueous phase, the shift in wavelength being significant. Nile Red has strong photochemical stability, an intense fluorescence emission peak, and relatively low cost. Perfect et al. further discloses that the method can be used to determine the size distribution of droplets of either the organic phase or the aqueous phase in the multiphase sample based on the image analysis. Perfect et al. further anticipates at-line and on-line applications.
Gerli et al. (U.S. Patent Application Publication No. 2009/0260767) describes a method of monitoring and controlling one or more types of hydrophobic contaminants in a papermaking process. The method utilizes the measurement with dyes, which are capable of fluorescing and interacting with the hydrophobic contaminants. Gerli et al. does not allow for the measurement of macrostickies or on-line monitoring. Gerli et al. provides for bulk characterization of a summary microstickies in a sample of filtered material.
Sakai (Japanese Patent Application Publication No. 2007/332467) proposed microscopic image processing of particles in a deinked pulp slurry for quantification of the particles. However, the process described in Sakai does not relate to the continuous monitoring with dye injection.
Accordingly, there is a need for complementing microstickies measurement technology with a comparable on-line method for macrostickies monitoring. Such a measurement method would allow a papemmaker/recycler to use lesser quality furnish and more recycled pulp than is currently able to be used. Desirably, the two measurement methods would allow for continuous on-line process adjustments in order to maximize process efficiency.